Writing instrument cap



FIG. 1 is a front elevational view of a writing instrument cap showing my new design;

FIG. 2 is a right-side elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a top plan view thereof; and

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a writing instrument cap, as shown and described. 